Some internal combustion propulsion engines, such as diesel engines which typically run unthrottled, have a compression release braking mechanism, sometimes simply called a compression release brake. A compression release brake functions to release air, or an air-exhaust mixture if exhaust gas is being recirculated to create the mixture, which reciprocating pistons have compressed within the engine cylinders during compression upstrokes of the pistons into an exhaust manifold of the engine so that energy used to compress the air or mixture is not recovered and used as a contribution to propulsion of the vehicle during ensuing downstrokes of the pistons.
When a motor vehicle is in motion after having been accelerated by its propulsion engine, and a driver of the vehicle ceases operating an accelerator control for the propulsion engine while road-engaging drive wheels of the vehicle continue to be coupled to the propulsion engine through a drivetrain, the propulsion engine begins to be driven by the road-engaging drive wheels through the drivetrain, rather than by combustion of fuel in the engine cylinders, and as a result, the load imposed on the drive wheels by the drivetrain and engine begins to decelerate the vehicle. If the engine has a compression release brake, the latter can be activated by the driver's operation of a compression release brake control to decelerate the vehicle more quickly than if the compression release brake is not activated. An example of such a control comprises an on-off switch for activating and de-activating the compression release brake and possibly a selector switch for selecting which engine cylinders will be used for engine braking. A control may also provide for engine braking to occur automatically upon the driver releasing the accelerator.
In an unthrottled turbocharged propulsion engine which has exhaust gas recirculation, the air/exhaust mixture from an intake manifold enters through an open cylinder intake valve or valves of a respective engine cylinder into the engine cylinder during an intake downstroke of a piston which reciprocates within the engine cylinder and is coupled by a connecting rod to a crankshaft of the engine. The mass flow into the respective engine cylinder is a function of pressure in the intake manifold which is created by a compressor (single- or multi-stage) of a turbocharger, i.e. is a function of boost created by a turbocharger compressor.
As the engine cycle for each engine cylinder transitions from an intake downstroke to a compression upstroke, the respective cylinder intake valve or valves operate from open to closed. Because one or more cylinder exhaust valves for each engine cylinder remain closed during the respective piston's compression upstroke, intake valve closing causes a volume of the air/exhaust mixture which has entered a respective engine cylinder during the piston downstroke to be trapped in the respective engine cylinder. As the respective piston upstrokes, it compresses the trapped volume. Kinetic energy of the moving vehicle provides the energy to compress the trapped volume, thereby contributing to vehicle deceleration.
In the absence of compression release braking, intake and exhaust valves for the respective engine cylinder would remain closed for substantially most of an ensuing downstroke of the respective piston after a compression upstroke, thereby allowing the energy of expansion of the trapped volume to force the respective piston downward and return energy through the drivetrain as a contribution to vehicle acceleration.
Activation of a compression release brake opens a respective engine cylinder to an exhaust manifold slightly in advance and/or during at least some portion of what would otherwise be an expansion power downstroke of the respective piston if combustion were occurring in the engine cylinder. Activation of the compression release brake causes energy imparted to the volume which was compressed during a compression upstroke to be dissipated to the exhaust manifold instead of being recovered and used to contribute to vehicle acceleration.
The purpose of activating a compression release engine brake is therefore to essentially eliminate contributions to vehicle acceleration which would otherwise occur during an expansion downstroke if the volume whose compression has contributed to vehicle deceleration during a compression upstroke were allowed to expand within the engine cylinder during the downstroke.
When travelling on roadways through mountainous regions, a vehicle may have no alternative but to operate at elevations significantly above sea level. The geography of such regions may compel roadway design to comprise significant grades along which a vehicle is likely to encounter both upgrades and downgrades. Equipping the propulsion engine of such a vehicle with a turbocharger enables the engine to develop increased torque and power useful for upgrade travel. Equipping the propulsion engine with a compression release brake enables the propulsion engine to decelerate the vehicle during downgrade travel either by itself or in conjunction with use of vehicle service brakes.